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Centre for Atmospheric Science

Aerosol Fluxes

Aerosol particles are emitted to the atmosphere from a wide variety of natural and anthropogneic sources, particles are also formed in situe through gas to particle conversion processes. They are mainly removed from the atmosphere through wet deposition and turbulent transport. More information about aerosol emission and deposition processes and their effects in the atmosphere can be found on our aerosol research pages. The so-called dry deposition of aerosols to the earth’s surface is a continuous and significant route for the transfer of chemical compounds, such as sulphate, nitrate, ammonium, oxidized and hydrocarbon organic compounds, and heavy metals from the atmosphere and plays an important role in the health or otherwise of many natural ecosystems. Flux measurements are used to study these emission and deposition processes. Eddy covariance techniques have been used to measure aerosol deposition to and emissions from grassland, forests, seaweed, agricultural land and urban areas. Aerosol flux measurements are often made in conjunction with aerosol characterisation and other measurements. Recent projects have focussed on aerosol flux measurements over seaweed, tropical forests and urban areas.

Typical eddy covariance systems for making measurements of aerosol fluxes consist of a sonic anemometer (turbulence) coupled with a fast response optical particle counter (aerosol size distribution) for size resolved aerosol flux measurements of accumulation mode aerosol. For ultrafine aerosol fluxes a pair of condensation particle counters (aerosol number) with different lower size cuts are used.

Fig 1. Aerosol Emission from Sea-Weed.
Fig 1. Aerosol Emission from Sea-Weed.

Fig 1. shows a Combined Eddy Covariance-Relaxed Eddy Accumulation System which was set up at the Station Biologique, Roscoff, France as part of the NERC RHAMBLE SOLAS project. This is designed to measure natural nano-particle emissions from macro-algae, ozone deposition fluxes to sea and sea-weed, VOC-Halocarbon fluxes (in collaboration with the University of York), Molecular Iodine fluxes, as well as sensible heat, latent heat and momentum fluxes. The system was designed so that it could be lowered and raised with the tide which can exceed 9 m at this site.

 

Fig 2. Aerosol Emissions from Cities. Average diurnal ultrafine (> 3nm) and fine (>11/15 nm) particle emission fluxes measured from towers using the eddy covariance technique on towers above four different cities, Edinburgh, Manchester, London & Gothenburg, in collaboration with CEH Edinburgh.
Fig 2. Aerosol Emissions from Cities. Average diurnal ultrafine (> 3nm) and fine (>11/15 nm) particle emission fluxes measured from towers using the eddy covariance technique on towers above four different cities, Edinburgh, Manchester, London & Gothenburg, in collaboration with CEH Edinburgh.




Fig 3: Sensible heat fluxes measured above three UK cities, Edinburgh, London & Manchester. Note the summertime heat fluxes from central Manchester. Due to the high density of steel/glass/concrete construction and low greenspace zones in central Manchester compared to the Edinburgh & London measurement sites, the heat fluxes rarely drop below zero in summer.
Fig 3: Sensible heat fluxes measured above three UK cities, Edinburgh, London & Manchester. Note the summertime heat fluxes from central Manchester. Due to the high density of steel/glass/concrete construction and low greenspace zones in central Manchester compared to the Edinburgh & London measurement sites, the heat fluxes rarely drop below zero in summer.

To study the process of aerosol movement to and from the earth surface and their potential to contribute to the above processes one must measure both the number and mass concentration of aerosols as a function of their size. To determine the rate at which aerosols are transported directly to or from a surface one must measure the vertical flux or the vertical velocity of the individual aerosols. At Manchester, and in collaboration with colleagues at the Centre for Ecology, Edinburgh, we have instrumentation and facilities that can achieve this.
Vertical fluxes of aerosols are measured using the eddy covariance technique which requires fast response aerosol spectrometers and turbulence sensors. Laser based spectrometers provide measurements of the concentration and size of aerosols on very rapid timescales and this information is correlated with the instantaneous vertical velocity using a high frequency ultrasonic turbulence sensor to provide the instantaneous vertical flux of the aerosol.

Fig 4. Some typical results of measurements of the deposition velocity of aerosols (both dry and wet) to different types of surfaces; grassland, forests and agricultural crops as a function of their size.

Fig 4. Some typical results of measurements of the deposition velocity of aerosols (both dry and wet) to different types of surfaces; grassland, forests and agricultural crops as a function of their size.

We conduct measurements of aerosol fluxes to and from a number of surfaces including
grasslands, forests, sea surfaces, from macro-algae (sea weed) and we also measure the
emission of ultrafine particles (UFP) from cities. These fluxes can be collected very rapidly over
many different meteorological conditions so that we can understand the processes that drive
them. The fluxes are then used to either test parameterisations of aerosol deposition rates for
use in regional pollution models or to scale emissions of anthropogenic and natural aerosols
from e.g. cities and forests.

Note: The data in fig4. is not presented in normalised Vds format due to lack of micrometeorological data from throughfall and SEM techniques. In addition throughfall may be associated with larger sizes (symbols represent size range).